Semiconductor device and method of manufacturing the same

ABSTRACT

A semiconductor device comprising a high breakdown voltage transistor and a low breakdown voltage transistor. The semiconductor device comprises a support substrate, an insulating layer formed on the support substrate, a high breakdown voltage transistor, a low breakdown voltage transistor, wherein the high breakdown voltage transistor is adjacent to a first isolation region having a depth that reaches the insulating layer, and the low breakdown voltage transistor is adjacent to a second isolation region having a depth that does not reach the insulating layer.

Japanese Patent Application No. 2003-051118 filed on Feb. 27, 2003 is hereby incorporated by reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to a semiconductor device equipped with MOS (Metal Oxide Semiconductor) transistors with different drain breakdown voltages that are provided on the same SOI (Silicon On Insulator) substrate.

2. Description of the Related Art

In recent years, mobile electronic appliances have been made smaller and lighter, making it essential to miniaturize ICs that are mounted in such electronic appliances. In particular, for an electronic appliance on which a liquid crystal display device is mounted, there are strong demands to reduce the chip area of a driver IC, which drives the liquid crystal display device, by mounting transistors with a low breakdown voltage for low voltage operations and transistors with a high breakdown voltage for high voltage operations on a same substrate (i.e., in the same chip).

However, to achieve a sufficiently high breakdown voltage, it is not possible to reduce the area of a transistor forming region for high breakdown voltage transistors. Accordingly, no matter how much low breakdown voltage transistors are miniaturized, it has not been possible to make large reductions in the chip area of a driver IC.

When high breakdown voltage transistors and low breakdown voltage transistors are formed on the same SOI substrate, it is necessary to change the structures of the transistors to make the transistors suitable for an SOI substrate so as to avoid the substrate floating effect and the like that are characteristic to SOI substrates. Therefore, it is not possible to apply design resources that have been produced for the case where transistors are formed on a bulk silicon substrate.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductor device equipped with a high breakdown voltage transistor and a low breakdown voltage transistor on the same substrate, and in particular to a semiconductor device which has a reduced area for a high breakdown voltage transistor region, which has a reduced overall size, and which eradicates characteristic effects, such as a substrate floating effect, of a SOI substrate in a low breakdown voltage transistor region, making it possible to apply conventional design resources, and a method of manufacturing such semiconductor device.

A semiconductor device according to the present invention comprises:

a support substrate;

an insulating layer formed on the support substrate;

a first semiconductor layer formed on the insulating layer;

a first high breakdown voltage transistor formed in the first semiconductor layer;

a second semiconductor layer formed on the insulating layer;

a second high breakdown voltage transistor formed in the second semiconductor layer;

a first isolation region that is provided between the first semiconductor layer and the second semiconductor layer and has a depth that reaches the insulating layer;

a third semiconductor layer formed on the insulating layer;

a first low breakdown voltage transistor provided in the third semiconductor layer;

a second low breakdown voltage transistor provided in the third semiconductor layer; and

a second isolation region that is formed in the third semiconductor layer between the first low breakdown voltage transistor and the second first low breakdown voltage transistor and has a depth that does not reach the insulating layer.

According to the present invention, the high breakdown voltage transistor is formed in a region that is surrounded by, or adjacent to, the first isolation region with a depth that reaches the insulating layer. This means that a wide isolation region that was conventionally required to achieve the breakdown voltage is no longer needed. Since the isolation region reaches the insulating layer, the occurrence of parasitic transistors that would sometimes be formed in a lower part of the isolation region can also be inhibited. Additionally, since there is no need to provide a high concentration dispersed layer as a guard ring, the area of the high breakdown voltage transistor region can be reduced. The low breakdown voltage transistor is formed so as to be surrounded by, or adjacent to, the second isolation region with a depth that does not reach the insulating layer, so that problems such as the substrate floating effect produced when an SOI substrate is used can be eliminated. As a result, even in a case where high breakdown voltage transistors and low breakdown voltage transistors are formed on the same substrate, it is possible to miniaturize a semiconductor device. In addition, conventional design resources can be applied to the low breakdown voltage transistors.

A method of manufacturing a semiconductor device according to the present invention comprises:

a step of preparing a substrate where a support substrate, an insulating layer, and a semiconductor layer are formed in that order;

a step of forming a first semiconductor layer, a second semiconductor layer, and a third semiconductor layer by forming, in the semiconductor layer, a first isolation region and a third isolation region with a depth that reaches the insulating layer;

a step of forming a second isolation region in the third semiconductor layer with a depth that does not reach the insulating layer;

a step of forming a first high breakdown voltage transistor in the first semiconductor layer;

a step of forming a second high breakdown voltage transistor in the second semiconductor layer;

a step of forming a first low breakdown voltage transistor in the third semiconductor layer; and

a step of forming a second low breakdown voltage transistor in the third semiconductor layer that is adjacent to the first low breakdown voltage transistor with the second isolation region in between.

According to the method of manufacturing a semiconductor device of the present invention, the high breakdown voltage transistor and the low breakdown voltage transistor are formed on semiconductor layers of the same thickness. The low breakdown voltage transistors are formed in regions surrounded by the second isolation region with a depth that does not reach the insulating layer, so that the characteristic effects of an SOI substrate, such as the substrate floating effect, are eradicated and conventional design resources can be applied. Also, since the high breakdown voltage transistors can be formed in semiconductor layers that are completely surrounded by the first isolation region, it is possible to manufacture a semiconductor device in which the occurrence of parasitic transistors, which would otherwise be formed in a lower part of the isolation region, is inhibited.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically showing a semiconductor device according to a present embodiment.

FIG. 2 is a cross-sectional view showing a step in a first method of manufacturing a semiconductor device.

FIG. 3 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 4 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 5 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 6 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 7 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 8 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 9 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 10 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 11 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 12 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 13 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 14 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 15 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 16 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 17 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 18 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 19 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 20 is a cross-sectional view showing a step in the first method of manufacturing a semiconductor device.

FIG. 21 is a cross-sectional view showing a step in a second method of manufacturing a semiconductor device.

FIG. 22 is a cross-sectional view showing a step in the second method of manufacturing a semiconductor device.

FIG. 23 is a cross-sectional view showing a step in the second method of manufacturing a semiconductor device.

FIG. 24 is a cross-sectional view showing a step in the second method of manufacturing a semiconductor device.

FIG. 25 is a cross-sectional view showing a step in a third method of manufacturing a semiconductor device.

FIG. 26 is a cross-sectional view showing a step in the third method of manufacturing a semiconductor device.

FIG. 27 is a cross-sectional view showing a step in a fourth method of manufacturing a semiconductor device.

FIG. 28 is a cross-sectional view showing a step in the fourth method of manufacturing a semiconductor device.

FIG. 29 is a cross-sectional view showing a step in the fourth method of manufacturing a semiconductor device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference to preferred embodiments thereof.

FIG. 1 is a cross-sectional view schematically showing a semiconductor device according to the present embodiment. The semiconductor device according to the present embodiment includes an SOI substrate 10 that has an insulating layer 10 b and a semiconductor layer 10 c formed in order as layers on a support substrate 10 a. The semiconductor layer 10 c is a single-crystal silicon layer. A high breakdown voltage transistor region 10HV and a low breakdown voltage transistor region 10LV are provided in the SOI substrate 10. The high breakdown voltage transistor region 10HV includes a P channel high breakdown voltage transistor region 10HVp and an N channel high breakdown voltage transistor region 10HVn. The low breakdown voltage transistor region 10LV includes a P channel low breakdown voltage transistor region 10LVp and an N channel low breakdown voltage transistor region 10LVn. A P channel high breakdown voltage transistor 100P is formed in the P channel high breakdown voltage transistor region 10HVp, and an N channel high breakdown voltage transistor 100N is formed in the N channel high breakdown voltage transistor region 10HVn. In the same way, a P channel low breakdown voltage transistor 200P is formed in the P channel low breakdown voltage transistor region 10LVp, and an N channel low breakdown voltage transistor 200N is formed in the N channel low breakdown voltage transistor region 10LVn.

That is, the P channel high breakdown voltage transistor 100P, the N channel high breakdown voltage transistor 100N, the P channel low breakdown voltage transistor 200P, and the N channel low breakdown voltage transistor 200N are formed on the same substrate (i.e., in the same chip). It should be noted that although only four transistors are shown in FIG. 1, this is merely for convenience and it should be obvious that a plurality of each type of transistor are formed on the same substrate.

In the regions where each transistor is formed, the thickness of the semiconductor layer 10 c is equal. Accordingly, since the semiconductor layer 10 c is formed above the support substrate 10 a whose thickness is even and the insulating layer 10 b whose thickness is even, the surface of the semiconductor layer 10 c is at the same level in each transistor formation region.

First, the high breakdown voltage transistor region 10HV will be described. At the boundary between the high breakdown voltage transistor region 10HV and the low breakdown voltage transistor region 10LV, first and third isolation regions 110 a, b are formed with a depth that reaches the insulating layer 10 b. That is, the third isolation region 110 b isolates the high breakdown voltage transistor region 10HV and the low breakdown voltage transistor region 10LV. As a result, the high breakdown voltage transistor region 10HV is surrounded by the third isolation region 110 b that is deep enough to reach the insulating layer 10 b.

The P channel high breakdown voltage transistor region 10HVp and the N channel high breakdown voltage transistor region 10HVn are provided in the high breakdown voltage transistor region 10HV. Between the adjacent high breakdown voltage transistor regions, first and third isolation regions 110 a, b are provided with a depth that reaches the insulating layer 10 b. That is, the first and third isolation regions 110 a, b are provided between the adjacent P channel high breakdown voltage transistor 100P and the N channel high breakdown voltage transistor 100N with a depth that reaches the insulating layer 10 b.

Next, the structures of the P channel high breakdown voltage transistor 100P and the N channel high breakdown voltage transistor 100N will be described.

The P channel high breakdown voltage transistor 100P includes a first gate insulating layer 60, a second gate insulating layer 112, a gate electrode 70, a P-type low concentration dopant layer 50, a sidewall insulating layer 72, and a P-type high concentration dopant layer 52.

The first gate insulating layer 60 is provided on an N-type well 32 that is a channel region. The second gate insulating layer 112 is provided above an offset region at both ends of the first gate insulating layer 60. It should be noted that as described later, the second gate insulating layer 112 is composed of a trench insulating layer 20 b, that is an offset insulating layer, and the first gate insulating layer 60 formed as layered films. The gate electrode 70 is formed on at least the first gate insulating layer 60. The P-type low concentration dopant layer 50 is the offset region. The sidewall insulating layer 72 is formed on a side surface of the gate electrode 70. The P-type high concentration dopant layer 52 is provided outside the sidewall insulating layer 72. The P-type high concentration dopant layer 52 is a source region or a drain region (hereinafter, “source/drain region”).

The N channel high breakdown voltage transistor 100N includes a first gate insulating layer 60, a second gate insulating layer 112, a gate electrode 70, an N-type low concentration dopant layer 40, a sidewall insulating layer 72, and an N-type high concentration dopant layer 42.

The first gate insulating layer 60 is provided on a P-type well 30 that is the channel region. The second gate insulating layer 112 is provided above an offset region at both ends of the first gate insulating layer 60. It should be noted that as described later, the second gate insulating layer 112 is composed of a trench insulating layer 20 b and the first gate insulating layer 60 formed as layered films. The gate electrode 70 is formed on at least the first gate insulating layer 60. The N-type low concentration dopant layer 40 is the offset region. The sidewall insulating layer 72 is formed on a side surface of the gate electrode 70. The N-type high concentration dopant layer 42 is provided outside the sidewall insulating layer 72. The N-type high concentration dopant layer 42 is a source/drain region.

First, the low breakdown voltage transistor region 10LV will be described. The P channel low breakdown voltage transistor region 10LVp and the N channel low breakdown voltage transistor region 10LVn are provided in the low breakdown voltage transistor region 10LV. Second isolation regions 210 with a depth that does not reach the insulating layer 10 b are provided between adjacent low breakdown voltage transistor regions. That is, the second isolation region 210 is provided between the P channel low breakdown voltage transistor 200P and the N channel low breakdown voltage transistor 200N with a depth that does not reach the insulating layer 10 b.

Next, the structure of each transistor will be described.

The N channel low breakdown voltage transistor 200N includes a gate insulating layer 62, a gate electrode 70, a sidewall insulating layer 72, an N-type low concentration dopant layer 41, and an N-type high concentration dopant layer 42.

The gate insulating layer 62 is provided on a P-type well 36 that is a channel region. The gate electrode 70 is formed on the gate insulating layer 62. The sidewall insulating layer 72 is formed on a side surface of the gate electrode 70. The N-type low concentration dopant layer 41 is an offset region. The N-type high concentration dopant layer 42 is provided outside the sidewall insulating layer 72. The N-type high concentration dopant layer 42 is a source/drain region.

The P channel low breakdown voltage transistor 200P includes a gate insulating layer 62, a gate electrode 70, a sidewall insulating layer 72, a P-type low concentration dopant layer 51, and a P-type high concentration dopant layer 52.

The gate insulating layer 62 is provided on the N-type well 34 that is a channel region. The gate electrode 70 is formed on the gate insulating layer 62. The sidewall insulating layer 72 is formed on a side surface of the gate electrode 70. The P-type low concentration dopant layer 51 is an offset region. The P-type high concentration dopant layer 52 is formed outside the sidewall insulating layer 72. The P-type high concentration dopant layer 52 is a source/drain region.

The advantages of the semiconductor device according to the present invention are as follows.

In the semiconductor device according to the present embodiment, the high breakdown voltage transistor region 10HV is surrounded by the third isolation region 110 b with a depth that reaches the insulating layer 10 b. The first and third isolation regions 110 a, b are formed between adjacent high breakdown voltage transistor regions with a depth that reaches the insulating layer 10 b. That is, completely isolated semiconductor layers 10 c can be formed for the high breakdown voltage transistors 100P and 100N. This means that it is possible to inhibit the formation of a parasitic MOS transistor that would conventionally be formed in a lower part of an isolation region. It also becomes unnecessary to provide a guard ring composed of a high concentration dispersed layer, and the area of the high breakdown voltage transistor region can be reduced.

In the semiconductor device according to the present embodiment, second isolation regions 210 with a depth that does not reach the insulating layer 10 b are provided between adjacent low breakdown voltage transistor regions. Accordingly, the low breakdown voltage transistors have approximately the same operation as bulk-type MOS transistors, characteristic effects of SOI substrates, such as the substrate floating effect, can be eradicated, and conventional design resources can be applied.

Next, a first method of manufacturing a semiconductor device will be described with reference to FIGS. 2 to 20. FIGS. 2 to 20 are cross-sectional views schematically showing steps in a first method of manufacturing the semiconductor device.

-   (1) As shown in FIG. 2, the semiconductor device according to the     present embodiment is formed of an SOI substrate 10 where an     insulating layer 10 b and a semiconductor layer 10 c are formed in     layers on a support substrate 10 a. A single-crystal silicon layer     can be used as the semiconductor layer 10 c. The thickness of the     semiconductor layer 10 c should preferably be 500 to 2000 nm. As     shown in FIG. 2, a first insulating layer 12 a is formed on the     semiconductor layer 10 c. A silicon oxide layer, a     silicon-oxynitride (SiON) layer, or the like is used as the first     insulating layer 12 a. The first insulating layer 12 a can be formed     by a method such as CVD.

Next, a stopper insulating layer 14 a is formed on the first insulating layer 12 a. The stopper insulating layer 14 a can be formed of a silicon nitride film. The stopper insulating layer 14 a can be formed by a method such as CVD. A resist layer R1 with a predetermined pattern is formed on the stopper insulating layer 14 a. The resist layer R1 has openings in regions where the first and third isolation regions 110 a, b are formed.

-   (2) Next, as shown in FIG. 3, the stopper insulating layer 14 a and     the first insulating layer 12 a are etched using the resist layer R1     (see FIG. 2) as a mask. After this, the semiconductor layer 10 c is     etched with the resist layer R1, the stopper insulating layer 14 a,     and the first insulating layer 12 a as a mask to form trenches 16 a.     When the trenches 16 a are formed, base parts of the trenches 16 a     are formed so as to reach the insulating layer 10 b. The etching of     the semiconductor layer 10 c can be performed by dry etching, for     example. -   (3) Next, as shown in FIG. 4, trench oxide films 18 a are formed on     surfaces of the trenches 16 a. These trench oxide films 18 a can be     formed by thermal oxidation, for example. The thickness of the     trench oxide films 18 a is 50 to 500 nm, for example.

Before the trench oxide films 18 a are formed, it is possible to etch end parts of the first insulating layer 12 a as necessary. By doing so, during the formation of the trench oxide films 18 a, it is possible to form the trench oxide films 18 a so as to be rounded at the upper end parts of the trenches 16 a. If the trench oxide films 18 a are formed so as to be rounded at the upper end parts of the trenches 16 a, no stepped parts are produced, therefore it becomes possible to favorably bury a trench insulating layer in a later step.

-   (4) Next, as shown in FIG. 5, a resist layer R2 with a predetermined     pattern is formed. This resist layer R2 has openings above a region     of the high breakdown voltage transistor region 10HV in which the     second gate insulating layer 112 is formed and openings above     regions of the low breakdown voltage transistor region 10LV in which     the second isolation regions 210 are formed. -   (5) Next, as shown in FIG. 6, the first insulating layer 12 a and     the stopper insulating layer 14 a are etched with the resist layer     R2 as a mask. Next, the semiconductor layer 10 c is etched with at     least the resist layer R2 as a mask. Trenches 16 b are formed in the     semiconductor layer 10 c by this etching. When the trenches 16 b are     formed, base parts of the trenches 16 b are formed so as to not     reach the insulating layer 10 b. The depth of the trenches 16 b can     be set at around 400 nm, for example. As a result of the patterning     by steps (2) to (5), the first insulating layer 12 a becomes a pad     layer 12, and the stopper insulating layer 14 a becomes a stopper     layer 14. After this, the resist layer R2 is removed by ashing. -   (6) Next, as shown in FIG. 7, trench oxide layers 18 b are formed on     surfaces of the trenches 16 b. The trench oxide layers 18 b can be     formed by thermal oxidation, for example. When doing so, the trench     oxide layers 18 b may be formed on the trench oxide films 18 a in     the trenches 16 a. Alternatively, the trench oxide layers 18 b may     be formed after the trench oxide films 18 a have been removed.

Next, an insulating layer 22 is formed so as to bury the trenches 16 a and the trenches 16 b. The insulating layer 22 may be thick enough to bury the trenches 16 a, 16 b and to cover the stopper layer 14. Next, as shown in FIG. 7, an SOG film 24 is applied onto the insulating layer 22 so as to form a flat surface.

-   (7) Next, as shown in FIG. 8, the SOG film 24 and the insulating     layer 22 are removed until an upper surface of the stopper layer 14     is exposed. The SOG film 24 and the insulating layer 22 are removed     using a method such as CMP. By doing so, trench insulating layers 20     a, 20 b are formed in the trenches 16 a, 16 b. As a result, the     first and third isolation regions 110 a, b and the second isolation     regions 210 are formed. Also, in this step, offset insulating layers     that are part of the second gate insulating layer 112 are formed in     the high breakdown voltage transistor region 10HV. It should be     noted that in the present embodiment, since the offset insulating     layers are formed by a method where element are isolated by     trenches, the offset insulating layers are called the trench     insulating layers 20 b. -   (8) Next, as shown in FIG. 9, the stopper layer 14 is removed. The     removal of the stopper layer 14 can be performed by wet etching     using hot phosphoric acid, for example. Next, a sacrificial oxide     film (not shown) is formed on an upper surface of the semiconductor     substrate 10. A silicon oxide film, for example, can be formed as     the sacrificial oxide film. In this case, the sacrificial oxide film     can be formed by thermal oxidation. -   (9) Next, as shown in FIG. 10, a P-type well 30 is formed in the     high breakdown voltage transistor region 10HV. More specifically, a     resist layer R3 with a predetermined pattern is formed, and the     P-type well 30 is formed by introducing P-type dopant ions into the     semiconductor layer 10 c with the resist layer R3 as a mask. After     this, the resist layer R3 is removed by ashing. -   (10) Next, as shown in FIG. 11, the N-type well 32 is formed in the     high breakdown voltage transistor region 10HV. First, a resist layer     R4 with a predetermined pattern is formed. The N-type well 32 is     formed by introducing N-type dopant, such as phosphorus or arsenic,     into the semiconductor layer 10 c once or a plurality of times with     the resist layer R4 as a mask. After this, the resist layer R4 is     removed by ashing. It should be noted that the steps (9) and (10)     may be performed in reverse order to that given in the present     embodiment. -   (11) Next, as shown in FIG. 12, a silicon nitride layer 26 is formed     on the entire surface of the high breakdown voltage transistor     region 10HV and the low breakdown voltage transistor region 10LV.

Next, dopant layers for offset regions of the source/drain regions are formed in the high breakdown voltage transistor region 10HV.

First, a resist layer RS that covers predetermined regions is formed. Dopant layers 40 a are formed by introducing a P-type dopant into the semiconductor layer 10 c with the resist layer R5 as a mask. After this, the resist layer R5 is removed by ashing.

-   (12) Next, as shown in FIG. 13, a resist layer R6 that covers     predetermined regions is formed. A P-type dopant is then introduced     into the semiconductor layer 10 c with the resist layer R6 as a     mask. By doing so, dopant layers 50 a for offset regions of the     source/drain region are formed in the P channel high breakdown     voltage transistor region 10HVp. -   (13) Next, as shown in FIG. 14, the dopant layers 40 a, 50 a are     dispersed by carrying out a heat treatment, thereby forming the low     concentration dopant layers 40, 50 that are the offset regions of     the high breakdown voltage transistors 100P, N. -   (14) Next, as shown in FIG. 15, a resist layer R7 is formed so as to     cover parts of the high breakdown voltage transistor region 10HV     aside from the regions in which the gate insulating layers of the     respective transistors are formed. The exposed silicon nitride layer     26 is then removed with the resist R7 as a mask. Next, channel     doping is performed as necessary in the high breakdown voltage     transistor region 10HV. The channel doping can be performed by the     following method, for example. First a resist layer (not shown) is     formed so as to cover the areas aside from the P-channel high     breakdown voltage transistor region 10HVp. A P-type dopant, for     example boron or the like, is then introduced with the resist layer     as a mask. After this, the resist layer is removed by ashing. Next,     a resist layer (not shown) is formed so as to cover the areas aside     from the N-channel high breakdown voltage transistor region 10HVn.     An N-type dopant, for example phosphorus or the like, is then     introduced with the resist layer as a mask. After this, the resist     layer is removed by ashing. -   (15) Next, as shown in FIG. 16, the first gate insulating layers 60     are formed in the high breakdown voltage transistor region 10HV. The     first gate insulating layers 60 can be formed by selective thermal     oxidation. The thickness of the first gate insulating layer 60 is     around 1,600 Å. Next, the remaining silicon nitride layer 26 is     removed. -   (16) Next, as shown in FIG. 17, the N-type well 34 and the P-type     well 36 are formed in the low breakdown voltage transistor region     10LV. More specifically, the N-type well 34 and the P-type well are     formed by forming a mask layer of a predetermined pattern using a     standard lithographic technique and introducing dopants of     predetermined conductivity types. Next, channel doping may be     performed as necessary. -   (17) Next, as shown in FIG. 18, a resist layer R8 is formed so as to     cover the region where the first gate insulating layers 60 are     formed in the high breakdown voltage transistor region 10HV, and the     exposed pad layer 12 is removed. This etching of the pad layer 12     can be performed by wet etching using fluoric acid, for example. -   (18) Next, as shown in FIG. 19, the gate insulating layer 62 for the     low breakdown voltage transistors is formed. The gate insulating     layer 62 is formed by thermal oxidation. The thickness of the gate     insulating layer 62 is 45 Å, for example. The gate insulating layer     62 is also formed in the high breakdown voltage transistor region     10HV.

Next, as shown in FIG. 19, a conductive layer 70 a is formed on the entire surface of the high breakdown voltage transistor region 10HV and the low breakdown voltage transistor region 10LV. A polysilicon layer, for example, is formed as the conductive layer 70 a. When a polysilicon layer is formed as the material of the conductive layer 70 a, n-type dopant is introduced into regions that become the gate electrodes of the N channel high breakdown voltage transistor 100N and the N channel low breakdown voltage transistor 200N (see FIG. 1), thereby lowering the resistance of the gate electrodes.

-   (19) Next, a resist layer (not shown) is formed with a predetermined     pattern. By patterning the polysilicon layer with the resist layer     as a mask, the gate electrodes 70 are formed, as shown in FIG. 20.

Next, low concentration dopant layers 41, 51 for the transistors 200P, N are formed in the low breakdown voltage transistor region 10LV. The low concentration dopant layers 41, 51 can be formed by forming mask layers using a standard lithography technique and introducing predetermined dopants.

-   (20) Next, an insulating layer (not shown) is formed on the entire     surface and by anisotropically etching this insulating layer, the     sidewall insulating layers 72 (see FIG. 1) are formed on the side     surfaces of the gate electrodes 70. Next, by introducing a P-type     dopant into a predetermined region of the P-channel high breakdown     voltage transistor region 10HVp and the P channel low breakdown     voltage transistor region 10LVp, the source/drain regions 52 are     formed outside the sidewall insulating layer 72 as shown in FIG. 1.     The formation of the P-type high concentration dopant layers 52 that     are the source/drain regions can be performed according to a known     method.

Next, by introducing an N-type dopant into predetermined regions of the N-channel high breakdown voltage transistor region 10HVn and the N channel low breakdown voltage transistor region 10LVn, the source/drain regions 42 are formed. The formation of the N-type high concentration dopant layers 42 that are the source/drain regions can be performed according to a known method.

As described above, the semiconductor device shown in FIG. 1 is formed.

-   (A) According to the method of manufacturing according to the     present embodiment, the high breakdown voltage transistors 100P, N     can be formed in regions isolated by the first and third isolation     regions 1110 a, b that reach the insulating layer 10 b. This means     that a wide isolation region that was conventionally required to     achieve the breakdown voltage is no longer necessary. The occurrence     of parasitic MOS transistors that are sometimes formed in the lower     part of the isolation region can also be inhibited. Additionally,     since there is no need to provide a high concentration dispersed     layer as a guard ring, the area of the high breakdown voltage     transistor region 10HV can be reduced. -   (B) The low breakdown voltage transistors 200P, N can be formed in     regions that are isolated by second isolation regions 210 with a     depth that does not reach the insulating layer 10 b. This means that     the characteristic effects of an SOI substrate, such as the     substrate floating effect, can be eradicated. In addition, it is     possible to apply conventional design resources in the low breakdown     voltage transistor region 10LV.

Next, a second method of manufacturing a semiconductor device will be described with reference to FIGS. 21 to 24. This second embodiment is an example where the method of forming the first and third isolation regions 110 a, b and the second isolation regions 210 differ from the first method of manufacturing a semiconductor device. FIGS. 21 to 24 are cross-sectional views schematically showing steps in the second method of manufacturing a semiconductor device. It should be noted that steps that can be performed in the same way as the first method of manufacturing a semiconductor device are shown using the same drawings and detailed description of such is omitted.

-   (1) First, as shown in FIG. 2, a first insulating layer 12 a is     formed on the semiconductor layer 10 c. Next, a stopper insulating     layer 14 a is formed on the first insulating layer 12 a. The first     insulating layer 12 a and the stopper insulating layer 14 a can be     formed in the same way as in the first embodiment. -   (2) Next, the resist layer R1 with a pattern shown in FIG. 21 is     formed on the stopper insulating layer 14 a. The resist layer R1 has     openings in regions in which the first and third isolation regions     110 a, b, the second isolation regions 210, and the second gate     insulating layer 112 of the high breakdown voltage transistor region     10HV are formed. Next, the first insulating layer 12 a is etched     using the resist layer R1 as a mask. After this, the semiconductor     layer 10 c is etched with the resist layer R1, the stopper     insulating layer 14 a, and the first insulating layer 12 a as a     mask. By doing so, trenches 16 a for the first and third isolation     regions 110 a, b and trenches 16 b for the second isolation regions     210 and the second gate insulating layer 112 are formed. Etching is     performed so that the depth of the trenches 16 a, b is around 300 to     1800 nm. Also, in this step, the first insulating layer 12 a and the     stopper insulating layer 14 a are patterned and the pad layer 12 and     the stopper layer 14 are formed. -   (3) Next, as shown in FIG. 22, the trench insulating layers 20 a, b     are formed so as to bury the trenches 16 a, b. The trench insulating     layers 20 a, b can be formed by the same method as steps (6) to (8)     in the first embodiment, for example.

Next, a silicon nitride layer 28 is formed on the semiconductor layer 10 c on which the trench insulating layers 20 a, b are formed. After this, a resist layer R2 with openings above the first and third isolation regions 110 a, b, that is, above the trenches 16 a, is formed on the silicon nitride layer 28.

-   (4) Next, as shown in FIG. 23, the silicon nitride layer 28 is     removed with the resist layer R2 as a mask. That is, the silicon     nitride layer 28 in the first and third isolation regions 110 a, b     is removed. In addition the trench insulating layer 20 a of the     trench 16 a is also removed. After this, the resist layer R2 is     removed by ashing. -   (5) Next, as shown in FIG. 24, thermal oxidation is performed with     the silicon nitride layer 28 as a mask. By doing so, the     semiconductor layer 10 c is oxidized at the first and third     isolation regions 110 a, b to form oxide films 20 c. By doing so,     the first and third isolation regions 110 a, b are formed with a     depth that reaches the insulating layer 10 b. -   (6) Next, the semiconductor device of the present embodiment can be     formed by performing the steps (9) to (20) of the first embodiment.

According to the method of manufacturing a semiconductor device according to the second embodiment, the first and third isolation regions 110 a, b can be made up of oxide films formed by thermal oxidation. This means that trenches with a depth that reaches the insulating layer 10 b are formed, and compared to the method where insulating layers are buried in these trenches, the amount of stress placed upon the semiconductor layer 10 c can be reduced. As a result, favorable first and third isolation regions 110 a, b can be formed.

Next, a third method of manufacturing a semiconductor device will be described with reference to FIGS. 25, 26. This third embodiment is an example where the method of forming the first and third isolation regions 110 a, b and the second isolation regions 210 differ to the method of manufacturing the first semiconductor device. FIGS. 25, 26 are cross-sectional views schematically showing steps in the third method of manufacturing a semiconductor device. It should be noted that steps that can be performed in the same way as the first and second methods of manufacturing a semiconductor device are shown using the same drawings and detailed description of such is omitted.

-   (1) First, as shown in FIG. 2, a first insulating layer 12 a is     formed on the semiconductor layer 10 c. Next, a stopper insulating     layer 14 a is formed on the first insulating layer 12 a. The first     insulating layer 12 a and the stopper insulating layer 14 a can be     formed in the same way as in the first embodiment. -   (2) Next, as shown in FIG. 21, a resist R1 with a predetermined     pattern is formed on the stopper insulating layer 14 a. The resist     layer R1 has openings in regions in which the first and third     isolation regions 110 a, b, the second isolation regions 210, and     the offset layer 112 of the high breakdown voltage transistor region     10HV are formed. Next, the semiconductor layer 10 c is etched using     the resist layer R1, the stopper insulating layer 14 a, and the     first insulating layer 12 a as a mask, to form the trenches 16 a, b.     Etching is performed so that the depth of the trenches 16 a, b is     around 300 to 1800 nm. Also, in this step, the first insulating     layer 12 a and the stopper insulating layer 14 a are patterned and     the pad layer 12 and the stopper layer 14 are formed. -   (3) Next, as shown in FIG. 25, a first trench oxide film 18 is     formed on the surfaces of the trenches 16 a, b. The first trench     oxide film 18 can be formed in the same way as step (3) in the first     embodiment. Next, a resist layer R2 with openings above the trenches     16 a is formed. Dopant ions are introduced into the semiconductor     layer 10 c in a base part of the trenches 16 a, with the resist     layer R2 as a mask. Here, it is possible to introduce chlorine, for     example, as the dopant ions. By doing so, defects are caused in the     semiconductor layer 10 c. This means that in the later step that     forms a thermal oxide film, it is possible to raise the formation     speed of thermal oxide films in the semiconductor layer 10 c in the     base parts of the trenches 16 a. After this, the resist layer R2 is     removed by ashing. Next, the first trench oxide film 18 formed on     the surfaces of the trenches 16 a, 16 b is removed. -   (4) Next, as shown in FIG. 26, a second trench oxide film 19 is     formed on the surfaces of the trenches 16 a, b. The second trench     oxide film 19 can be formed by a method such as thermal oxidation.     Due to this thermal oxidation step, the semiconductor layer 10 c at     the base parts of the trenches 16 a is thermally oxidized to become     the oxide films 20 c. That is, parts of the first and third     isolation regions 110 a, b, whose depth reaches the insulating layer     10 b, are formed.

Next, trench insulating layers 20 a, b are formed in the trenches 16 a, b. These trench insulating layers 20 a, b can be formed in the same way as in steps (6), (7) of the first embodiment. After this, the stopper layer 14 is removed, the first and third isolation regions 110 a, b are formed in the high breakdown voltage transistor region 10HV, and the second isolation regions 210 are formed in the low breakdown voltage transistor region 10LV.

-   (5) Next, the semiconductor device shown in FIG. 1 can be formed by     performing the steps (9) to (20) of the method of manufacturing the     first semiconductor device.

According to the method of manufacturing a third semiconductor device, the trenches 16 a, b can be formed simultaneously, so that the number of steps can be reduced. Also, by introducing dopant into the base parts of the trenches 16 a, the formation speed of the thermal oxide films can be increased. As a result, at the same time as the second trench oxide film 19 is formed, oxide films which are thicker than the second trench oxide film 19 and reach the insulating layer 10 b are formed in the base parts of the trenches 16 a.

Next, a fourth method of manufacturing a semiconductor device will be described with reference to FIGS. 27 to 29. This fourth embodiment is an example where the method of forming the first and third isolation regions 110 a, b and the second isolation regions 210 differ to the first method of manufacturing a semiconductor device. FIGS. 27 to 29 are cross-sectional views schematically showing steps in a fourth method of manufacturing a semiconductor device. It should be noted that steps that can be performed in the same way as the first method of manufacturing a semiconductor device are shown using the same drawings and detailed description of such is omitted.

-   (1) First, as shown in FIG. 2, a first insulating layer 12 a is     formed on the semiconductor layer 10 c. Next, a stopper insulating     layer 14 a is formed on the first insulating layer 12 a. The first     insulating layer 12 a and the stopper insulating layer 14 a can be     formed in the same way as in the first embodiment. -   (2) Next, as shown in FIG. 27, a resist layer R1 with openings above     the second isolation regions 210 of the low breakdown voltage     transistor region 10LV, and above the second gate insulating layer     112 of the high breakdown voltage transistor region 10HV is formed     on the stopper insulating layer 14 a. Next, the stopper insulating     layer 14 a and the first insulating layer 12 a are etched with the     resist layer R1 as a mask. After this, the trenches 16 b are etched     with the resist layer R1, the stopper insulating layer 14 a, and the     first insulating layer 12 a as a mask. Etching is performed so that     the depth of the trenches 16 b is around 400 nm. After this, the     resist layer R1 is removed by ashing. Next, the trench oxide films     18 are formed on the surfaces of the trenches 16 b. The trench oxide     films 18 are formed by thermal oxidation, for example. -   (3) Next, as shown in FIG. 28, the resist layer R2 with a     predetermined pattern is formed. The resist layer R2 has openings     above the first and third isolation regions 110 a, b of the high     breakdown voltage transistor region 10HV. Next, the stopper     insulating layer 14 a and the first insulating layer 12 a are etched     with the resist layer R2 as a mask. After this, the semiconductor     layer 10 c is etched with the resist layer R2, the stopper     insulating layer 14 a, and the first insulating layer 12 a as a mask     to form the trenches 16 a. Etching is performed until the depth of     the trenches 16 a is such that an insulating film 20 c, which is     formed at the same time as a subsequent formation of oxide films,     that is, the trench oxide films 19, definitely reaches the     semiconductor layer 10 c.

Next, dopant ions are introduced into the base parts of the trenches 16 a. By doing so, defects are caused in the single crystal silicon layer that constructs the semiconductor layer 10 c. After this, the resist layer R2 is removed by ashing. Next, the trench oxide films 18 formed on the surfaces of the trenches 16 b are removed.

-   (4) Next, as shown in FIG. 29, trench oxide films 19 are formed on     the surfaces of the trenches 16 a, b. The trench oxide films 19 can     be formed by thermal oxidation, for example. The thickness of the     trench oxide films 19 is around 50 to 100 nm, but thicker films may     be formed. As a result of this step, the semiconductor layer 10 c     that is in the base parts of the trenches 16 a is thermally oxidized     to become the oxide films 20 c. By doing so, part of the first and     third isolation regions 110 a, b that reach the insulating layer 10     b can be formed.

Next, the trench insulating layers 20 a, b are formed in the trenches 16 a, b. The trench insulating layers 20 a, b can be formed in the same way as the (6), (7) of the first embodiment. After this, the stopper layer 14 is removed, the first and third isolation regions 110 a, b are formed in the high breakdown voltage transistor region 10HV, and the second isolation regions 210 are formed in the low breakdown voltage transistor region 10LV.

-   (6) Next, the semiconductor device shown in FIG. 1 can be formed by     performing the steps (9) to (20) of the first method of     manufacturing a semiconductor device.

According to the fourth method of manufacturing a semiconductor device, in the first and third isolation regions 110 a, b, the formation of the trenches 16 a can be combined with the formation of the oxide films 20 c. This means that the difference in depths between the trenches 16 a and the trenches 16 b can be reduced, and the trench insulating layers 20 a, b can be favorably buried.

Also, by introducing dopant into the base parts of the trenches 16 a it is possible to increase the formation speed of the thermal oxide layer. This means that the semiconductor layer 10 c at the base parts of the trenches 16 a can be formed of a thicker oxide layer than regions of the semiconductor layer in which dopant is not introduced, and the first and third isolation regions 110 a, b can be formed with a depth that reaches the insulating layer 10 b.

It should be noted that the present invention is not limited to the above embodiments, and that modifications may be made within the range covered by the gist of the invention.

For example, in the first method of manufacturing a semiconductor device, it is possible to form the trench insulating layers 20 a, b as follows. First, as shown in FIG. 6, the trenches 16 a, b are formed. After this, an insulating layer is formed as thickly as possible to bury only the trenches 16 b. This insulating layer can be formed according to HDP or CVD. An SOG film is then formed so as to bury parts of the trenches 16 a that are not filled with the insulating layer. Next, the trench insulating layers 20 a, b can be formed by removing the SOG film by CMP until the stopper layer 14 is exposed.

Also, the second method of manufacturing a semiconductor device can be modified as follows. First, the trenches 16 a are formed for the first and third isolation regions 110 a, b. At this point, the trenches 16 a are positioned deeper than the trenches 16 b for the second isolation regions 210 and are formed so as to not reach the insulating layer 10 b. Next, an oxide film can be formed on the surfaces of the trenches 16 a by thermal oxidation, and the first and third isolation regions 110 a, b can be formed with a depth that reaches the insulating layer 10 b. After this, the second isolation regions 210 and the offset layer 112 are formed. In this state, by appropriately controlling the ratio of the width of the trenches 16 a to the thickness of the semiconductor layer 10 c at the base parts of the trenches 16 a, it is possible to further relieve stress. For example, the ratio of the width of a trench 16 a to the thickness of the semiconductor layer 10 c at the base part of the trench 16 a should preferably be 2:1.

Although embodiments of the present invention have been described, the present invention is not limited to these embodiments and modifications are possible within the range covered by the gist of the invention. For example, offset insulating layers that are parts of the second isolation regions 210 and the second gate insulating layer 112 were described as being formed in the above embodiments by a method where elements are isolated by trenches, but can also be formed using LOCOS isolation or semi-recessed LOCOS isolation. 

1-25. (canceled)
 26. A semiconductor device comprising: a support substrate; an insulating layer formed on the support substrate; a first semiconductor layer formed on the insulating layer, the first semiconductor layer including a first semiconductor region of a first transistor; a first gate insulating film of the first transistor formed on the first semiconductor region, the first gate insulating film having a first thickness; a first isolation region formed adjacent to the first semiconductor layer, the first isolation region having a first depth that reaches the insulating layer; a second isolation region formed adjacent to the first semiconductor layer, the second isolation region having a second depth that reaches the insulating layer; a second semiconductor layer formed on the insulating layer, the second semiconductor layer being adjacent to the second isolation region, the second semiconductor layer including a second semiconductor region of a second transistor; a second gate insulating film of the second transistor formed on the second semiconductor region, the second gate insulating film having a second thickness; a third isolation region formed adjacent to the second semiconductor layer, the third isolation region having a third depth that reaches the insulating layer; a third semiconductor layer formed on the insulating layer, the third semiconductor layer including a third semiconductor region of a third transistor; a third gate insulating film of the third transistor on the third semiconductor region, the third gate insulating film having a third thickness that is smaller than the first thickness or the second thickness; a forth isolation region formed adjacent to the third semiconductor layer, the fourth isolation region having a fourth depth that is shallower than the first depth, the second depth or the third depth; a fifth isolation region formed adjacent to the third semiconductor layer, the fifth isolation region having a fifth depth that is shallower than the first depth, the second depth or the third depth; a fourth semiconductor layer formed on the insulating layer, the fourth semiconductor layer including a fourth semiconductor region of a fourth transistor; a fourth gate insulating film of the fourth transistor formed on the fourth semiconductor region, the fourth gate insulating film having a fourth thickness that is smaller than the first thickness or the second thickness; a sixth isolation region formed adjacent to the fourth semiconductor layer, the sixth isolation region having a sixth depth that is shallower than the first depth, the second depth or the third depth; and wherein only one first transistor is formed between the first isolation region and the second isolation region; wherein only one second transistor is formed between the second isolation region and the third isolation region.
 27. A semiconductor device comprising: a first semiconductor layer, the first semiconductor layer including a first semiconductor region of a first transistor; a first gate insulating film formed on the first semiconductor region, the first gate insulating film having a first thickness; a first isolation region formed adjacent to the first semiconductor layer, the first isolation region having a first depth; a second isolation region formed adjacent to the first semiconductor layer, the second isolation region having a second depth; a second semiconductor layer, the second semiconductor layer including a second semiconductor region of a second transistor; a second gate insulating film formed on the second semiconductor region, the second gate insulating film having a second thickness that is smaller than the first thickness; a third isolation region formed adjacent to the second semiconductor layer, the third isolation region having a third depth that is shallower than the first depth or the second depth; and a fourth isolation region formed adjacent to the second semiconductor layer, the fourth isolation region having a fourth depth that is shallower than the first depth or the second depth; wherein only one first transistor is formed between the first isolation region and the second isolation region.
 28. A semiconductor device according to claim 27, wherein only one first transistor is formed between the first isolation region and the second isolation region at a predetermined cross sectional view.
 29. A semiconductor device according to claim 27, further including, a support substrate; an insulating layer on the support substrate, the insulating layer being located below the first semiconductor layer and the second semiconductor layer.
 30. A semiconductor device according to claim 27, further including, a third semiconductor layer formed adjacent to the second isolation region, the semiconductor layer including a third semiconductor region of a third transistor; a third gate insulating film formed on the third semiconductor region, the third gate insulating film having a third thickness; a fifth isolation region formed adjacent to the third semiconductor layer, the fifth isolation region having a fifth depth; a fourth semiconductor layer formed adjacent to the fourth isolation region, the fourth semiconductor layer including a fourth semiconductor region of a fourth transistor; a fourth gate insulating film formed on the fourth semiconductor region, the fourth gate insulating film having a fourth thickness that is smaller than the first thickness or the third thickness; a sixth isolation region formed adjacent to the fourth semiconductor layer, the sixth isolation region having a sixth depth that is shallower than the first depth, the second depth or the fifth depth.
 31. A semiconductor device according to claim 30, wherein only one third transistor is formed between the second isolation region and the fifth isolation region.
 32. A semiconductor device according to claim 30, wherein no other isolation region is formed between the second isolation region and the fifth isolation region.
 33. A semiconductor device comprising: a first semiconductor layer, the first semiconductor layer including a first semiconductor region of a first transistor; a first gate insulating film formed on the first semiconductor region, the first gate insulating film having a first thickness; a first isolation region formed adjacent to the first semiconductor layer, the first isolation region having a first depth; a second isolation region formed adjacent to the first semiconductor layer, the second isolation region having a second depth; a second semiconductor layer, the second semiconductor layer including a second semiconductor region of a second transistor; a second gate insulating film formed on the second semiconductor region, the second gate insulating film having a second thickness that is smaller than the first thickness; a third isolation region formed adjacent to the second semiconductor layer, the third isolation region having a third depth that is shallower than the first depth or the second depth; and a fourth isolation region formed adjacent to the second semiconductor layer, the fourth isolation region having a fourth depth that is shallower than the first depth or the second depth; wherein no other isolation region is formed between the first isolation region and the second isolation region.
 34. A semiconductor device according to claim 33, wherein no other isolation region is formed between the first isolation region and the second isolation region. at a predetermined cross sectional view.
 35. A semiconductor device according to claim 33, further including, a support substrate; an insulating layer on the support substrate, the insulating layer being located below the first semiconductor layer and the second semiconductor layer.
 36. A semiconductor device according to claim 33, further including, a third semiconductor layer formed adjacent to the second isolation region, the semiconductor layer including a third semiconductor region of a third transistor; a third gate insulating film formed on the third semiconductor region, the third gate insulating film having a third thickness; a fifth isolation region formed adjacent to the third semiconductor layer, the fifth isolation region having a fifth depth; a fourth semiconductor layer formed adjacent to the fourth isolation region, the fourth semiconductor layer including a fourth semiconductor region of a fourth transistor; a fourth gate insulating film formed on the fourth semiconductor region, the fourth gate insulating film having a fourth thickness that is smaller than the first thickness or the third thickness; a sixth isolation region formed adjacent to the fourth semiconductor layer, the sixth isolation region having a sixth depth that is shallower than the first depth, the second depth or the fifth depth.
 37. A semiconductor device according to claim 36, wherein only one third transistor is formed between the second isolation region and the fifth isolation region.
 38. A semiconductor device according to claim 36, wherein no other isolation region is formed between the second isolation region and the fifth isolation region.
 39. A semiconductor device comprising: a first semiconductor layer, the first semiconductor layer including a first semiconductor region of a first transistor, a first dopant region of the first transistor and a second dopant region of the first transistor a first gate insulating film of the first transistor formed on the first semiconductor region, the first gate insulating film having a first thickness; a first isolation region formed adjacent to the first dopant region, the first isolation region having a first depth; a second isolation region formed adjacent to the second dopant region, the second isolation region having a second depth; a second semiconductor layer, the second semiconductor layer including a second semiconductor region of a second transistor, a third dopant region of the second transistor and a fourth dopant region of the second transistor; a second gate insulating film of the second transistor formed on the second semiconductor region, the second gate insulating film having a second thickness that is smaller than the first thickness; a third isolation region formed adjacent to the third dopant region, the third isolation region having a third depth that is shallower than the first depth or the second depth; and a fourth isolation region formed adjacent to the fourth dopant region, the fourth isolation region having a fourth depth that is shallower than the first depth or the second depth.
 40. A semiconductor device according to claim 39, wherein the first dopant region is a source or a drain of the first transistor; wherein the second dopant region is the source or the drain of the first transistor.
 41. A semiconductor device according to claim 39, further including, a support substrate; an insulating layer on the support substrate, the insulating layer being located below the first semiconductor layer and the second semiconductor layer.
 42. A semiconductor device according to claim 39, further including, a third semiconductor layer formed adjacent to the second isolation region, the semiconductor layer including a third semiconductor region of a third transistor; a third gate insulating film formed on the third semiconductor region, the third gate insulating film having a third thickness; a fifth isolation region formed adjacent to the third semiconductor layer, the fifth isolation region having a fifth depth; a fourth semiconductor layer formed adjacent to the fourth isolation region, the fourth semiconductor layer including a fourth semiconductor region of a fourth transistor; a fourth gate insulating film formed on the fourth semiconductor region, the fourth gate insulating film having a fourth thickness that is smaller than the first thickness or the third thickness; a sixth isolation region formed adjacent to the fourth semiconductor layer, the sixth isolation region having a sixth depth that is shallower than the first depth, the second depth or the fifth depth.
 43. A semiconductor device according to claim 42, wherein only one third transistor is formed between the second isolation region and the fifth isolation region.
 44. A semiconductor device according to claim 42, wherein no other isolation region is formed between the second isolation region and the fifth isolation region.
 45. A method of manufacturing a semiconductor device, the method comprising the steps of: preparing a substrate including a support substrate, an insulating layer, and a semiconductor layer; forming a first isolation region and a third isolation region with a depth that reaches the insulating layer such that the semiconductor layer is divided a first semiconductor layer, a second semiconductor layer, and a third semiconductor layer; forming a second isolation region in the third semiconductor layer with a depth that does not reach the insulating layer; forming a first high breakdown voltage transistor in the first semiconductor layer; forming a second high breakdown voltage transistor in the second semiconductor layer; forming a first low breakdown voltage transistor in the third semiconductor layer; and forming a second low breakdown voltage transistor in the third semiconductor layer that is adjacent to the first low breakdown voltage transistor with the second isolation region therebetween.
 46. A method of manufacturing a semiconductor device according to claim 45, wherein the second isolation region is formed by trench isolation.
 47. A method of manufacturing a semiconductor device according to claim 45, wherein the second isolation region is formed by LOCOS.
 48. A method of manufacturing a semiconductor device according to claim 45, wherein the second isolation region is formed by semi-recessed LOCOS.
 49. A method of manufacturing a semiconductor device according to claim 45, wherein the first and third isolation regions are made up of oxide films formed by thermal oxidation.
 50. A method of manufacturing a semiconductor device according to claim 45, wherein the first and third isolation regions are formed by etching first and second trenches, and further including the step of oxidizing base parts of the trenches to form oxide films, wherein the formation of the trenches is combined with the formation of the oxide film.
 51. A method of manufacturing a semiconductor device according to claim 45, wherein the first and second isolation regions are formed made up of oxide films formed by etching first and second trenches.
 52. A method of manufacturing a semiconductor device according to claim 51, wherein the first and second trenches are formed simultaneously.
 53. A method of manufacturing a semiconductor device according to claim 45, wherein the steps of forming the first and second high breakdown voltage transistors, further comprising the steps of: forming an offset insulating layer above an offset region; and forming a first gate insulating layer above at least a channel region and the offset region, thereby forming a second gate insulating layer above the offset region, the second gate insulating layer including the offset insulating layer and the first gate insulating layer.
 54. A method of manufacturing a semiconductor device according to claim 53, wherein the steps of forming the offset insulating layer and the second isolation region are performed by the same step.
 55. A method of manufacturing a semiconductor device according to claim 53, wherein the second isolation region is formed by trench isolation.
 56. A method of manufacturing a semiconductor device according to claim 53, wherein the second isolation region is formed by LOCOS.
 57. A method of manufacturing a semiconductor device according to claim 53, wherein the second isolation region is formed by semi-recessed LOCOS.
 58. A method of manufacturing a semiconductor device, the method comprising the steps of: preparing a substrate including a support substrate and an insulating layer; forming a first isolation region having a depth that reaches the insulating layer; forming a second isolation region having a depth that does not reach the insulating layer; forming a first high breakdown voltage transistor in a region adjacent to the first isolation region; and forming a first low breakdown voltage transistor in a region adjacent to the second isolation region. 